Harbour seals that come on land in the Wadden Sea area are dependent on the Wadden Sea and adjacent North Sea waters where they feed, sleep and mate. Seals are, however, difficult to observe and count at sea, but accurate counts in the Wadden Sea can be done when they haul out on the sandbanks to rest, moult or breed. To obtain data on the population size in the Wadden Sea and on Helgoland, surveys of harbour seals and their pups, are coordinated between the three Wadden Sea countries: Denmark, Germany, and the Netherlands.